The technique for recording and reproducing information by irradiating a laser beam that is narrowed down into a minute spot onto a thin film that has been formed on a transparent substrate is well known. Recently, various examinations have been carried out enthusiastically in order to increase the quantity of information that can be processed per optical information recording medium by utilizing a technique for recording and reproducing information signals having a high signal quality by irradiating a high-energy beam such as a laser beam to a thin film formed on a substrate. The methods can be divided into two major categories.
A first method is to increase the quantity of information per unit area. A spot diameter of a laser beam is made smaller by shortening the wavelength of a laser beam or by increasing the numerical aperture of an objective lens that gathers the laser beam, thus enabling a smaller mark to be recorded and reproduced. Consequently, the recording density in the circumferential direction and radial recording density in a disk increase and the quantity of information that can be processed per medium increases. Furthermore, in order to improve the recording density in the circumferential direction, mark edge recording in which the length of a recording mark becomes information, and in order to improve the radial recording density, land and groove recording in which information is recorded in both a groove and a land for guiding a laser beam, have been invented and applied vigorously. As such techniques for a high density recording and reproducing progress, a thin film material that is suitable for the techniques and a disk structure using the material also have been developed.
As a second method, a medium having a multilayer structure in which the quantity of information processed per recording medium is doubled by laminating a plurality of layers recording and reproducing information and a method of recording and reproducing in the medium have been proposed (for example, in Japanese Patent Application No. Hei 07-82248). Many thin film materials are also proposed as a recording material suitable for the recording medium having a multilayer structure, but a recording medium in which a favorable recording property can be obtained when only one layer is used is basically used as it is in many cases.
In an optical information recording medium (wherein the information layer is a single layer) utilizing a technique for recording and reproducing information signals having a high signal quality by irradiating a high-energy beam such as a laser beam, a thin film material whose main component is TeO.sub.x (o&lt;x&lt;2), that is a mixture of Te and TeO.sub.2, is provided on a substrate (Unexamined Japanese Patent Publication (Tokkai Sho) 50-46317). This kind of recording media can change a reflectance greatly in irradiating an optical beam for reproducing.
However, it takes some time in TeO.sub.x till a signal is saturated after recording, i.e. till a laser beam irradiated part in a recording thin film is crystallized sufficiently. This is not suitable for a recording medium in which a rapid response is required as in the case of a data file for a computer in which, for example, data are recorded in a disk and then verified after one rotation, or the like.
A recording medium in which, for example, Pd is added to TeO.sub.x as a third element in order to alleviate the disadvantage mentioned above is proposed in Unexamined Japanese Patent Publication (Tokkai Sho) 61-68296. The Te and the Pd act as a metal sensing a beam and the TeO.sub.x acts for maintaining an oxidation resistance. The TeO.sub.x is present as a matrix (sea) component and the Te and the Pd are present as an island component. It is conceivable that the Pd functions as a crystalline nucleus that promotes crystal growth of the Te at the time of irradiating a laser beam in a TeO.sub.x thin film. According to this function, crystal grains of Te or a Te--Pd alloy in which crystallization is further advanced are generated at high speed. As a result, it enables a crystallization recording at high speed, thus obtaining a rapid response as mentioned above. Furthermore, the moisture resistance of the TeO.sub.x thin film is not damaged, since the Pd has a high oxidation resistance.
However, a further improvement in recording density has come to be required according to the recent tendency toward mass storage of information. Consequently, a development of a recording medium that can correspond to high density recording using an optical system having a short wavelength/high NA has come to be required. That is to say, deterioration of a recording and reproducing property such as, for example, decrease of a C/N ratio and increase of jitter was found when trying to record information in a higher density recording than the experimental condition shown in the Publication mentioned above in many parts in a composition range of an optical information recording medium mentioned in the Publication in which Pd is added into TeO.sub.x. The C/N ratio mentioned above means a ratio of carrier/noise in a signal having a specific frequency.
The reason for the deterioration is considered to occur as follows. In the case of higher density recording and reproducing using the same optical system, a sufficient recording property cannot be obtained, if a thermal conductivity of a recording thin film does not fall in a predetermined range. That is, when the thermal conductivity of a recording thin film is too low, the heat is difficult to spread from the part heated by a laser beam and a recording mark can not be enlarged even if recording power is increased. Consequently, the sensitivity is low and the C/N ratio also tends to decrease. On the contrary, when the thermal conductivity of a recording thin film is too high, the heat is easy to spread from the part heated by a laser beam and a recording mark is enlarged by increasing the recording power a little. Consequently, the sensitivity is high and the C/N ratio also tends to increase. However, the edge of the recording mark easily becomes blurry. Adjacent marks begin to run together when increasing the recording power of a laser beam even slightly beyond the optimum power and the C/N ratio decreases. Therefore, the power margin is narrow and there is a problem in a practical use. It is conceivable that in recording and reproducing in the same optical system the problem becomes significant, as the density becomes high by narrowing the space between marks. Even if a high C/N ratio is obtained, it does not always mean that few bit errors occur. Thermal interference tends to be generated between the recording marks, for example, in the case where a recording thin film has a high thermal conductivity as mentioned above. As a result, it is conceivable that the position of the recording marks to be detected changes and many bit errors occur even in the cam of a high reflectance change and a high C/N ratio. It can be considered that this becomes more prominent in a mark edge recording method that is in wide use recently. An evaluation of a jitter is used as a means for evaluating the amount of bit errors relatively easily. The jitter means a deviation on a time base between an original signal for recording and a reproduced signal.
In the present specification, the value obtained by dividing the sum (.sigma..sub.sum) of standard deviation of a jitter in each signal by a window width (T) of signal detection is indicated as a "jitter (.sigma..sub.sum /T)", and the value is determined by measurement.
For example, it is well known that a jitter of 12.8% or less is corresponding to a bit error rate of 10.sup.-4 or less in the case of assuming that the deviation on a time base mentioned above is distributed normally.
The recording conditions in the Publication mentioned above are described as follows: a laser wavelength of 830 nm; a wavelength limitation of 0.8 .mu.m; a rotational frequency of 1800 rpm; a recording position (radius) of 75 mm; and a recording frequency of 5 MHz. In this case, it is conceivable by taking the technical background at the time the invention was made in Shouwa 61 (1986) into consideration that a marking position recording method was used. Therefore, the shortest marking space is 2.83 .mu.m and the bit length b is determined as 1.89 .mu.m by dividing the shortest marking space by a bit density of 1.5 under the condition of a linear velocity of 14.1 m/s calculated from the radial position and the rotational frequency mentioned above.
It is conceivable that the wavelength limitation of 0.8 .mu.m mentioned in the identical Publication has been calculated using a lens NA of 0.5, when taking it into consideration that beam intensity of a laser beam can be approximated by a Gaussian distribution usually and that it has been common at that time to define a diameter in which a beam intensity is a half of that in a spot center as the wavelength limitation.
When beam intensity of a laser beam can be approximated by a Gaussian distribution and a spot diameter d is defined as a diameter in which the beam intensity is 1/e of that in a spot center, the spot diameter d is 1.01 .mu.m. Thus, the ratio (b/d) of a bit length b for a spot diameter d is 1.87. In the Publication mentioned above, it is mentioned that a high C/N ratio, at least 50 dB under the condition of b/d=1.87 and about 60 dB depending on the composition, is obtained.
However, the recording capacity required for a recording medium has doubled in a few years recently and the ratio b/d mentioned above is necessary to be lowered greatly. The case of recording information signals of about 4 times of CD-ROM, i.e. 2.6 gigabyte, in a disk in which a film is formed on a substrate having the same size as a CD-ROM is considered as an example. In this case, the shortest mark length is 0.62 .mu.m under the condition of a groove pitch of 1.48 .mu.m when recording in both a groove and a land in a mark edge recording method. Consequently, the bit length b is 0.41 .mu.m, calculated by dividing the shortest mark length by a bit density of 1.5. In the case of using an optical system (for example, a wavelength of 680 nm and a NA of 0.6) that has been technically established recently and mass-produced, the spot diameter d is 0.59 .mu.m. Therefore, the b/d is about 0.6 under the condition mentioned above. The b/d is remarkably low under this condition compared to that in the Publication mentioned above, and an preferable recording property is not always exhibited under this condition even in the case of using the recording medium in the Publication as it is.
Therefore, in order to obtain a preferable recording property having a high C/N ratio and a small jitter in a wide power margin in recording and reproducing with a small b/d and higher density, the composition of the recording film mentioned in the Publication can not be applied as it is. It is considered that the composition of the recording thin film suitable for that is necessary to be reevaluated in the relationship with recording conditions.
Moreover, in obtaining a multilayer-recording medium in which a plurality of layers for recording and reproducing information are laminated, recording sensitivity is a critical issue, and transmittance and reflectance of a film also are necessary to be designed most suitably. That is to say, in a medium having a multilayer structure, especially the first information layer that is the first layer from the side into which a laser beam enters is required to have a high transmittance for recording and reproducing information with a sufficient power for the second information layer that is the second layer from the side. A high reflectance is also required to obtain a sufficient quantity of reflected light also from the first information layer itself. Consequently, absorption of the first information layer is lowered inevitably, resulting in difficulty of securing sufficient recording sensitivity. The level of recording density in each layer in a medium having a multilayer structure capable of recording that has been reported is lower than the current level mentioned above. As a result, an increase in the quantity of information that can be processed per recording medium can not be attained. It is also difficult to be attained using a semiconductor laser that can be mass-produced, since, for example, the laser power is required to be at least 20 mW in order to obtain a sufficient reflected light by recording signals in both of the two layers.
As far as the conventional example of a TeO.sub.x type recording thin film is concerned, it has been confirmed that the material is suitable for use as a medium capable of recording only for a thick film having a film thickness of about 120 nm as described in the reference or the like, which is very thick and hardly transmits a laser beam. Therefore, in order to use a film as the first information layer of a recording medium having a multilayer structure as mentioned above, the film is necessary to transmit a laser beam greatly. The recording property in such an area is unknown, and it is necessary to attain high density and high sensitivity for the preferable recording property.